Andrew Mwangura, an official of the East African Seafarers Assistance Programme, said that the negotiations between the Ukrainian owners and the pirates were still going on over the phone.
"It will take a while because the Ukrainian company may not pay all the money," Mwangura said.
US navy ships are watching the ship, whose capture has also raised questions over the destination of its cargo.
Kenya says the T-72 tanks, grenade-launchers and ammunition were for its military but the US navy believes they were headed for south Sudan via the port city of Mombasa.
Taking advantage of chaos on shore, where an insurgency has raged for nearly two years, Somali pirates have seized more than 30 ships this year and attacked many more.
Most attacks have been in the Gulf of Aden between Yemen and north Somalia, a major global sea artery used by about 20,000 vessels a year heading to and from the Suez Canal.
Mwangura said the ship was owned by a Panama-based firm and managed by the Ukrainians. There are 20 crew on board after one Russian died of illness. Most of the crew are Ukrainians, besides two Russians and a Latvian.
In an interesting twist, Kenyan police on Wednesday said they have detained Mwangura.
"We have been looking for him since yesterday, but we finally have him. He has been too vocal on the media, we want him to share with us what he knows of these pirates," a police official told AFP.
"We just want to question him on a few issues. It appears he knows more on the ship," the official said.
Mwangura earlier said the tanks on board the ship were meant for Sudan and not Kenya.